[#12] Use buffer pool for message encoding

```
goos: linux
goarch: amd64
pkg: git.frostfs.info/TrueCloudLab/zapjournald
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
                                     │      1       │                  2                  │
                                     │    sec/op    │   sec/op     vs base                │
Logger/standard/no_fields-8             450.3n ± 2%   452.9n ± 3%        ~ (p=0.138 n=10)
Logger/standard/application_fields-8    610.4n ± 1%   608.4n ± 1%        ~ (p=0.325 n=10)
Logger/standard/journald_fields-8       595.8n ± 5%   604.5n ± 1%        ~ (p=0.075 n=10)
Logger/journald/no_fields-8            1809.0n ± 4%   900.4n ± 1%  -50.23% (p=0.000 n=10)
Logger/journald/application_fields-8    2.037µ ± 5%   1.087µ ± 1%  -46.62% (p=0.000 n=10)
Logger/journald/journald_fields-8       2.054µ ± 4%   1.119µ ± 1%  -45.52% (p=0.000 n=10)
geomean                                 1.036µ        753.1n       -27.33%

                                     │        1        │                  2                   │
                                     │      B/op       │    B/op     vs base                  │
Logger/standard/no_fields-8               0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
Logger/standard/application_fields-8      128.0 ± 0%     128.0 ± 0%        ~ (p=1.000 n=10) ¹
Logger/standard/journald_fields-8         64.00 ± 0%     64.00 ± 0%        ~ (p=1.000 n=10) ¹
Logger/journald/no_fields-8            1206.000 ± 0%     5.000 ± 0%  -99.59% (p=0.000 n=10)
Logger/journald/application_fields-8     1494.0 ± 0%     133.0 ± 0%  -91.10% (p=0.000 n=10)
Logger/journald/journald_fields-8       1478.00 ± 0%     85.00 ± 0%  -94.25% (p=0.000 n=10)
geomean                                              ²               -83.36%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                     │       1       │                  2                   │
                                     │   allocs/op   │ allocs/op   vs base                  │
Logger/standard/no_fields-8             0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10) ¹
Logger/standard/application_fields-8    1.000 ± 0%     1.000 ± 0%        ~ (p=1.000 n=10) ¹
Logger/standard/journald_fields-8       1.000 ± 0%     1.000 ± 0%        ~ (p=1.000 n=10) ¹
Logger/journald/no_fields-8            29.000 ± 0%     1.000 ± 0%  -96.55% (p=0.000 n=10)
Logger/journald/application_fields-8   30.000 ± 0%     2.000 ± 0%  -93.33% (p=0.000 n=10)
Logger/journald/journald_fields-8      31.000 ± 0%     3.000 ± 0%  -90.32% (p=0.000 n=10)
geomean                                            ²               -75.38%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean
```

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

encoder.go

@@ -0,0 +1,58 @@
+package zapjournald
+
+import (
+	"bytes"
+	"strings"
+
+	"go.uber.org/zap/buffer"
+)
+
+var pool = buffer.NewPool()
+
+func writeFieldBytes(buf *buffer.Buffer, name string, value []byte) {
+	buf.Write([]byte(name))
+	if bytes.ContainsRune(value, '\n') {
+		// According to the format, if the value includes a newline
+		// need to write the field name, plus a newline, then the
+		// size (64bit LE), the field data and a final newline.
+
+		buf.Write([]byte{'\n'})
+		appendUint64Binary(buf, uint64(len(value)))
+	} else {
+		buf.Write([]byte{'='})
+	}
+	buf.Write(value)
+	buf.Write([]byte{'\n'})
+}
+
+func writeField(buf *buffer.Buffer, name string, value string) {
+	buf.Write([]byte(name))
+	if strings.ContainsRune(value, '\n') {
+		// According to the format, if the value includes a newline
+		// need to write the field name, plus a newline, then the
+		// size (64bit LE), the field data and a final newline.
+
+		buf.Write([]byte{'\n'})
+		// 1 allocation here.
+		// binary.Write(w, binary.LittleEndian, uint64(len(value)))
+		appendUint64Binary(buf, uint64(len(value)))
+	} else {
+		buf.Write([]byte{'='})
+	}
+	buf.WriteString(value)
+	buf.Write([]byte{'\n'})
+}
+
+func appendUint64Binary(buf *buffer.Buffer, v uint64) {
+	// Copied from https://github.com/golang/go/blob/go1.21.3/src/encoding/binary/binary.go#L119
+	buf.Write([]byte{
+		byte(v),
+		byte(v >> 8),
+		byte(v >> 16),
+		byte(v >> 24),
+		byte(v >> 32),
+		byte(v >> 40),
+		byte(v >> 48),
+		byte(v >> 56),
+	})
+}

zapjournald.go

@@ -2,6 +2,7 @@ package zapjournald
 
 import (
 	"fmt"
+	"strconv"
 
 	"github.com/ssgreg/journald"
 	"go.uber.org/zap/zapcore"
@@ -70,14 +71,10 @@ func (core *Core) Write(entry zapcore.Entry, fields []zapcore.Field) error {
 		return err
 	}
 
-	// Generate the message.
-	buffer, err := core.encoder.EncodeEntry(entry, fields)
-	if err != nil {
-		return fmt.Errorf("failed to encode log entry: %w", err)
-	}
-	defer buffer.Free()
+	b := pool.Get()
+	defer b.Free()
 
-	message := buffer.String()
+	writeField(b, "PRIORITY", strconv.Itoa(int(prio)))
 
 	structuredFields := maps.Clone(core.contextStructuredFields)
 	for _, field := range fields {
@@ -85,9 +82,28 @@ func (core *Core) Write(entry zapcore.Entry, fields []zapcore.Field) error {
 			structuredFields[field.Key] = getFieldValue(field)
 		}
 	}
+	for k, v := range structuredFields {
+		switch v := v.(type) {
+		case []byte:
+			writeFieldBytes(b, k, v)
+		case string:
+			writeField(b, k, v)
+		default:
+			writeField(b, k, fmt.Sprint(v))
+		}
+	}
+
+	// Generate the message.
+	buffer, err := core.encoder.EncodeEntry(entry, fields)
+	if err != nil {
+		return fmt.Errorf("failed to encode log entry: %w", err)
+	}
+	defer buffer.Free()
+
+	writeFieldBytes(b, "MESSAGE", buffer.Bytes())
 
 	// Write the message.
-	return core.j.Send(message, prio, structuredFields)
+	return core.j.WriteMsg(b.Bytes())
 }
 
 // Sync flushes buffered logs (not used).